Fe₃O₄@SiO₂ core–shell nanoparticles were synthesized and used to label human mesenchymal stem cells (hMSCs) for in vitro and in vivo magnetic resonance imaging study. The diameter of the nanoparticles is 24–30 nm with a Fe₃O₄ core of ～8 nm and a SiO₂ shell of ～8 nm. Transverse relaxivity of the nanoparticles dispersed in water is measured to be ～106 mM^？1 s^？1. After incubation with hMSCs for 12 h at a concentration of 100 μg Fe/mL, cellular uptake of Fe₃O₄@SiO₂ is 20–100 pg Fe/cell, which are located predominantly in the cytoplasm of cells. This level of uptake exhibits no significant influence on hMSCs' viability and differentiation. In vitro imaging of Fe₃O₄@SiO₂-labeled hMSCs evenly distributed in agarose gel yields single cell sensitivity at 11.7 T. In vivo imaging of Fe₃O₄@SiO₂-labeled hMSCs injected into the left brain hemisphere of nude mice yields imaging sensitivity of ～130 hMSCs.

Keywords:MRI; hMSCs; Fe3O4@SiO2; Single cell sensitivity